Realizing molecular pixel system for full-color fluorescence reproduction: RGB-emitting molecular mixture free from energy transfer crosstalk

Article abstract of DOI:10.1021/ja404256s

A full-color molecular pixel system is realized for the first time using simple mixtures composed of RGB-emitting excited-state intramolecular proton transfer (ESIPT) dyes, each of which has delicately tailored Stokes shift and independent emission capability completely free from energy transfer crosstalk between them. It is demonstrated that the whole range of emission colors enclosed within the RGB color triangle on the CIE 1931 diagram is predictable and conveniently reproducible from the RGB molecular pixels not only in the solution but also in the polymer film. It must be noted that mixing ratios to reproduce the desired color coordinates can be precisely calculated on the basis of additive color theory according to their molecular pixel behavior.

Full text of DOI:10.1021/ja404256s

Article
pubs.acs.org/JACS
Realizing Molecular Pixel System for Full-Color Fluorescence
Reproduction: RGB-Emitting Molecular Mixture Free from Energy
Transfer Crosstalk
Ji Eon Kwon,† Sanghyuk Park,‡ and Soo Young Park*,†
†Center for Supramolecular Optoelectronic Materials and WCU Hybrid Materials Program, Department of Materials Science and
Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-744, Korea
‡Department of Chemistry, Kongju National Univeristy, 56 Gongjudaehak-ro, Gongju, Chungnam 314-701, Korea
S
* Supporting Information
ABSTRACT: A full-color molecular pixel system is realized
for the first time using simple mixtures composed of RGB-
emitting excited-state intramolecular proton transfer (ESIPT)
dyes, each of which has delicately tailored Stokes shift and
independent emission capability completely free from energy
transfer crosstalk between them. It is demonstrated that the
whole range of emission colors enclosed within the RGB color
triangle on the CIE 1931 diagram is predictable and
conveniently reproducible from the RGB molecular pixels
not only in the solution but also in the polymer film. It must be
noted that mixing ratios to reproduce the desired color coordinates can be precisely calculated on the basis of additive color
theory according to their molecular pixel behavior.
Dexter, and trivial). Because the excited state of donor and the
ground state of acceptor are highly populated under
illumination, the energy transfer between conventional
fluorescent dyes cannot be blocked at all. Accordingly,
enormous research efforts to achieve multicolor luminescence
including white emission have been directed to the elaborate
tuning of mixing ratios between energy transferable dyes by
trial and error in a specific condition based on gels,14−19 metal
complexes,20,21 metal−organic frameworks (MOFs),22,23 self-
assembled nanoparticles,24−27 copolymers,28−31 polymeric
nanoparticles,32−34 silica nanoparticles,35−38 and supramolecu-
lar polymers,38−40 instead of completely blocking the energy
transfer between different molecules. This approach, however,
requires repetitive and time-consuming experiments to check
all of emission spectrum for every single mixing ratio of dyes
one by one. Needless to say, it is almost impossible to predict
and reproduce the emission colors of the mixture precisely in a
certain circumstance.
Very recently, unconventional multicolor-emitting molecules
showing frustrated energy transfer crosstalk between the
emitting centers (i.e., molecular pixel behavior) have been
reported by several groups including us, which exploit excited-
state intramolecular proton transfer (ESIPT) process.41−47
ESIPT relies on an enol−keto phototautomerization process of
molecules that possess an intramolecular hydrogen bond
between a proton-donor (i.e., −OH) and a proton-acceptor
INTRODUCTION
■
During the past decades, extensive attention has been paid to
multicolor luminescent materials due to their application
potential in flexible full-color displays,1−3 in the next-generation
lighting sources,4−6 and in bioimaging agents to decipher
multiple biological events simultaneously.7−10 The method to
generate multicolor luminescence is to blend either three
primary colors (i.e., red, green, and blue) or two comple-
mentary colors (i.e., blue and yellow), while keeping their
emission sources independent and free from energy transfer
crosstalk through spatial separation of them.11 In fact, this
strategy has been widely used as RGB-emitting pixel system in
full-color displays.12 According to the additive color theory, the
entire range of emission colors in the color gamut can be
predictable and conveniently reproducible by control of
emission intensity ratios between micrometer-sized RGB
color pixels. Contrary to bulk pixels, however, reproducing
color in a molecular scale by simple mixing of different RGB
chromophores is problematic due to the prevalent and
unavoidable energy transfer between them.13 Because the
efficiency of energy transfer is exponentially enhanced as the
components concentration increases, overwhelmingly domi-
nant emission from the lower band gap components is
routinely observed even when they are present in very small
amounts.
This energy transfer crosstalk, which is a main obstacle
against the realization of “molecular pixel”, proceeds through
the interaction between an excited-state donor and a ground-
Received: April 29, 2013
Published: July 11, 2013
state acceptor, irrespective of its actual mechanism (i.e., Forster,
̈
© 2013 American Chemical Society
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Figure 1. (a) Schematic representation of the ESIPT photocycle. (b) Chemical structures of the ESIPT chromophores used in this study and a
schematic illustration of the frustrated energy transfer.
(i.e., −N) in a five- or a six-membered ring.48−50 The
EXPERIMENTAL SECTION
■
photoinduced tautomerization transiently generates a keto form
Materials. Materials obtained from commercial suppliers were used
without further purification unless otherwise stated. DCM dye was
purchased from Sigma-Aldrich. All glassware, syringes, magnetic
(i.e., O and −NH−), which typically produces fluorescence
with a large Stokes shift (6000−12 000 cm−1). Reverse proton
stirring bars, and needles were thoroughly dried in a convection
oven. Reactions were monitored using thin layer chromatography
(TLC) with commercial TLC plates (silica gel 60 F254, Merck Co.).
Silica gel column chromatography was performed with silica gel 60
(particle size 0.063−0.200 mm, Merck Co.). 1H and 13C NMR spectra
were recorded on a Bruker Avance 300 spectrometer. High-resolution
mass (HRMS) spectra were acquired by employing JEOL JMS-600W/
JMS-700GC, and elemental analyses were performed on a CE
Instrument EA1110 instrument.
transfer in the ground state from the keto form (K) to the
original enol form (E) completes a four-level cyclic proton
transfer reaction (E → E* → K* → K → E, see Figure 1a).
Because the ground-state K exists only transiently due to the
subsequent backward proton transfer, the population of
ground-state K as a potential energy acceptor is negligible. As
a result, the energy transfer between the ESIPT molecules (i.e.,
interaction between the K* of donor and K of acceptor) can be
totally frustrated.
The frustrated energy transfer between ESIPT chromophores
enables them to behave as independent color pixels, even
though they are located close to each other within molecular
length scale. Even by covalently linking a pair of comple-
mentary color-emitting imidazole ESIPT derivatives as
subpixels (blue and yellow), we have already demonstrated a
two-color “molecular pixel”.42,44 Various photo- and electro-
luminescence colors from blue through white to yellow could
be successfully generated from this molecular pixel emitter by
modulating either photoexcitation wavelength or charge carrier
balance. For the realistic full-color reproduction, however, we
still have to develop a modular system composed of three
primary color molecular pixels.
In the present work, we demonstrate a simple and
straightforward mixture system, which consists of RGB-
emitting molecular pixels harnessing ESIPT for full-color
fluorescence reproduction. It is specifically shown that the
whole range of emission colors enclosed within the RGB color
triangle is precisely predicted and conveniently reproduced
from the RGB molecular pixels by adjusting mixing ratios of
them, which is based on the exact additive color theory, not
only in the solutions but also in the polymer films.
Synthesis of m-MHPI, 5-Methoxy-2-(1,4,5-triphenyl-1H-imi-
dazol-2-yl)phenol. 2-Hydroxy-4-methoxybenzaldehyde (1.00 g, 6.57
mmol) and aniline (0.60 mL, 6.57 mmol) were dissolved in acetic acid
(100 mL) and stirred for 1 h at room temperature. Benzil (1.38 g, 6.57
mmol) and ammonium acetate (3.55 g, 46.00 mmol) were added
subsequently. The mixture was heated at 120 °C overnight. After
termination of the reaction, the dark solution was poured into copious
amounts of water. After neutralization, the mixture was filtered and
washed with water. Reprecipitation in methanol from dichloromethane
1
solution afforded white powder (1.64 g, yield = 60%). H NMR (300
MHz, CDCl3, δ): 13.68 (s, 1H), 7.53 (dd, J = 8.1, 1.4 Hz, 2H), 7.42−
7.32 (m, 3H), 7.30−7.10 (m, 10H), 6.61 (d, J = 2.6 Hz, 1H), 6.42 (d, J
= 8.9 Hz, 1H), 6.04 (dd, J = 8.9, 2.6 Hz, 1H), 3.76 (s, 3H). 13C NMR
(75 MHz, CDCl3, δ): 161.17, 160.56, 145.51, 137.40, 134.99, 133.41,
131.56, 130.17, 130.02, 129.77, 129.31, 128.98, 128.62, 128.53, 128.44,
127.08, 106.51, 105.45, 102.10, 55.38. HRMS (FAB+, m/z): [M + H]+
calcd for C28H23N2O2, 419.1760; found, 419.1761. Anal. Calcd for
C28H22N2O2: C, 80.36; H, 5.30; N, 6.69. Found: C, 80.36; H, 5.39; N,
6.65.
Synthesis of p-MHPIC, 4-Methoxy-2-(1-phenyl-1H-
phenanthro[9,10-d]imidazol-2-yl)phenol. 2-Hydroxy-5-methoxy-
benzaldehyde (1.07 g, 7.00 mmol) and aniline (0.64 mL, 7.00 mmol)
were dissolved in acetic acid (100 mL) and stirred for 1 h at room
temperature. 9,10-Phenanthrenequinone (1.46 g, 7.00 mmol) and
ammonium acetate (3.78 g, 49.00 mmol) were added subsequently.
The mixture was heated at 120 °C overnight. After termination of the
reaction, the dark solution was poured into copious amounts of water.
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Figure 2. Optical properties of the ESIPT molecules. Absorption (filled circles) and normalized photoluminescence (empty circles) spectra of m-
MHPI (blue), HPI (sky blue), p-MHPIC (green), and HPNO (red) (a) in solution (CHCl3, c = 1 × 10−5 M) and (b) in dye-doped PMMA films (c
= 1 wt %) at room temperature. The excitation wavelength was 330 nm for the solutions and 340 nm for the films, respectively. (c) Normalized
emission spectra of the ESIPT dye ternary mixtures with molar ratios of m-MHPI:p-MHPIC:HPNO = 1:3:5 in chloroform solution at different
concentrations (1 × 10−6 to 1 × 10−3 M). The dashed lines indicate the estimated spectra of the mixture (black) and the corresponding dye
component: m-MHPI (blue), p-MHPIC (green), and HPNO (red), respectively. The excitation wavelength was 330 nm. (d) Normalized emission
spectra of the binary mixture with molar ratios of m-MHPI/DCM = 1:1 in chloroform solution at different concentrations (1 × 10−6 to 1 × 10−3 M).
Arrows indicate direction of spectral changes with increasing dye concentrations. The excitation wavelength was 320 nm.
After neutralization, the mixture was filtered and washed with water.
The organic compounds were reprecipitated in methanol from
dichloromethane solution, which was dried over anhydrous MgSO4
and concentrated. Silica gel column purification with EtOAc:CHCl3:n-
hexane (1:10:50, v/v/v) gave a white powder (1.24 g, yield = 45%).
1H NMR (300 MHz, CDCl3, δ): 13.32 (s, 1H), 8.78 (d, J = 8.3 Hz,
1H), 8.71 (d, J = 8.2 Hz, 2H), 7.83−7.60 (m, 7H), 7.53 (t, J = 7.4 Hz,
1H), 7.31−7.21 (m, 1H), 7.06 (dd, J = 8.5, 4.2 Hz, 2H), 6.82 (dd, J =
8.9, 2.9 Hz, 1H), 6.34 (d, J = 2.9 Hz, 1H), 3.29 (s, 3H). 13C NMR (75
MHz, CDCl3, δ): 153.61, 151.33, 148.41, 139.48, 134.72, 131.12,
130.77, 129.69, 129.47, 128.63, 127.68, 127.21, 126.70, 126.25, 126.00,
125.45, 124.38, 123.38, 122.77, 121.01, 118.95, 118.83, 112.65, 109.71,
55.32. HRMS (FAB+, m/z): [M + H]+ calcd for C28H21N2O2,
417.1603; found, 417.1606. Anal. Calcd for C28H20N2O2: C, 80.75; H,
4.84; N, 6.73. Found: C, 80.77; H, 4.80; N, 6.73.
Measurements. UV−vis absorption spectra were collected on a
Shimadzu UV-1650PC spectrophotometer at room temperature.
Fluorescence spectra were obtained by using a Varian Cary Eclipse
fluorescence spectrophotometer. The photoluminescence quantum
efficiencies (Φr, PL) in solution were obtained using quinine sulfate in
1.0 N sulfuric acid as a reference. On the other hand, the Φab,PL values
in thin films on a quartz plate were measured using a 3.2-in. integrating
sphere equipped on a QM-40 spectrophotometer (Photon Technol-
ogy International, Canada). Time-resolved fluorescence lifetime
experiments were performed through the time-correlated single
photon counting (TCSPC) methods by using a FluoTime 200
instrument (Picoquant, Germany). A 342 nm pulsed LED with
repetition rate of 10 MHz was used as an excitation source.
Fluorescence decay profiles were analyzed by FluoFit Pro software,
using exponential fitting models through deconvolution employing
instrumental response functions (IRF).
Calculations. All density functional theory (DFT) calculations
were carried out in the gas phase using the Gaussian 09 quantum-
chemical package.51 The geometry optimizations for the ground state
of imidazole and oxazole derivatives were performed using B3LYP
functionals with 6-31G(d,p) basis set.52 Vibration frequency
calculations at the same level were performed for the obtained
structures to confirm the global minimum.
RESULTS AND DISCUSSION
■
To construct a molecular pixel system for full-color
reproduction, three primary color-emitting ESIPT chromo-
phores having suitable Commission Internationale de l’Eclair-
age (CIE) chromaticity coordinate as well as high fluorescence
quantum yield are ideally desirable. The hydroxy-substituted
tetraphenylimidazole derivatives are typical ESIPT molecules,
which emit strong fluorescence not merely in the solution but
more intensely in the solid state. It is rationalized that the steric
crowding of the four phenyl rings prevents tight stacking of the
active chromophores, thereby maintaining appropriate inter-
molecular distances to suppress intermolecular nonradiative
decay pathways.53−58 A highly efficient amplified spontaneous
emission as well as organic light-emitting diodes (OLEDs) was
successfully demonstrated with the ESIPT-active imidazole
molecules incorporating multifunctional substituents.42,44,53,54
Although we have already developed and demonstrated a series
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Table 1. Photophysical Properties of the Imidazole Derivatives in CHCl3 Solution and PMMA Doped Thin Films
a
b
e
solution
λmax,abs (nm) ε (L mol−1 cm−1
film
fluorescence lifetime
τ2 (ns)
c
d
)
λmax,em (nm)
Φr,PL
λmax,abs (nm) λmax,em (nm) Φab,PL
τ1 (ns)
τavg(ns)
m-MHPI
HPI
317
319
371
2.23 × 104
1.91 × 104
2.17 × 104
2.15 × 104
461
472
513
0.062
0.15
317
318
371
332
447
462
0.27
0.37
0.44
2.14 (71.9%)
2.42 (24.7%)
1.59 (25.4%)
0.63 (69.9%)
3.07 (28.1%)
3.93 (75.3%)
5.04 (74.6%)
1.56 (30.1%)
2.40
3.55
4.16
p-MHPIC
HPNO
0.032
0.0057
506
333
628
422, 623
0.014
0.91
a
b
c
d
10 μM in CHCl3. 1 wt % of dye doped in PMMA. The relative PL quantum yields were measured using quinine sulfate as a reference. The
e
absolute PL quantum yields were measured using an integrating sphere with QM40 spectrophotometer. The fluorescence lifetime values of the films
were measured at 440, 460, 520, and 620 nm for m-MHPI, HPI, p-MHPIC, and HPNO, respectively.
of nine imidazole ESIPT derivatives whose emission wave-
length was widely tuned from 470 to 630 nm,59 they still lack
one exhibiting green K* emission (500 nm < λmax < 550 nm).
In addition, the highest energy emitting fluorophore, 2-(1,4,5-
triphenyl-1H-imidazol-2-yl)phenol (HPI, λmax = 472 nm,
Figures 1b and 2a) in the series does not show deep blue
emission but only sky blue emission, which is rather insufficient
for the full-color reproduction.
nm) and green (λmax = 513 nm) fluorescence emission in the
solution, which were blue- (11 nm) and red-shifted (41 nm) as
compared to that of nonsubstituted HPI (λmax = 472 nm),
respectively. HPNO exhibited red (λmax = 628 nm) K*
emission with a very weak E* emission band at around 410
nm in the solution. The relative quantum yields (Φr,PL) of the
solutions are 0.062, 0.032, and 0.0057 for the blue (m-MHPI),
green (p-MHPIC), and red (HPNO) emissions, respectively.
On the other hand, a slight hypsochromic shift of the
emission was observed in the imidazole dye-doped PMMA
films, whereas their absorption was almost identical to those in
the solution. Nevertheless, the imidazole molecules still
fluoresce three primary RGB colors in the polymer films,
respectively. Furthermore, it is noteworthy that these RGB
imidazole derivatives exhibited a large Stokes shifted emission
(>130 nm) between 440 and 630 nm originating from their
respective keto tautomers, while their absorption spectra were
all localized within UV region, leading to almost no spectral
overlap between their absorption and emission bands.
To identify a given set of chromophores as molecular pixels,
the emission spectrum of individual chromophore molecule
should be unaffected by the presence of other chromophores
when they are mixed together even to the very high
concentrations. To confirm the independent emission, three
sets of ternary mixture solutions were prepared in molar ratios
of m-MHPI:p-MHPIC:HPNO = 5:3:1, 1:1:1, and 1:3:5,
respectively, at various total chromophore concentrations
from 1 μM to 1 mM. As expected for the molecular pixels
showing frustrated energy transfer crosstalk, the measured
emission spectra of the mixture solutions could be precisely
estimated by simply adding the emission spectra of individual
components weighted to their mixing ratios (see the scattered
symbols and dashed lines in Figure 2c and Figure S3 in the
Supporting Information). It should be noted that spectral shape
of a given mixture solution is independent of its total
chromophore concentration, providing unambiguous evidence
for frustrated energy transfer between the emitting components
in the mixture solutions. The main principle and working
mechanism of frustrated energy transfer between ESIPT
imidazole species have been described in full detail in our
earlier publication.42
Therefore, two imidazole-based ESIPT derivatives were
newly designed and synthesized in this work for the blue and
green emitting molecular pixels by introducing a donor
substituent such as methoxy group into the phenol ring of
HPI. As previously reported by us,42 DFT calculation results
show that π-electrons of both the highest occupied and the
lowest unoccupied molecular orbitals (HOMO and LUMO)
are extensively delocalized over the whole molecule in the case
of enol form of HPI (see Supporting Information Figure S1).
Yet, the occurrence of proton transfer, forming a keto form,
brings about π-electron deficiency at a specific position
especially in the phenol ring. In the case of keto form of
HPI, π-electron density of the HOMO at meta-position to the
ketone group is lower than the other positions. In contrast, π-
electron density of the LUMO is deficient at para-position to
the ketone group of the keto form of HPI. It is, therefore,
expected that adding a methoxy group at the meta-position
should raise the LUMO energy of the keto form while hardly
affecting the HOMO level, to give an overall hypsochromic
shift of the K* emission. Conversely, a similar substitution at
the para-position should raise only the HOMO energy of the
keto form, resulting in a bathochromic shift of the K* emission
(see Supporting Information Figure S2).
According to the classic lophine synthesis,60 both m-MHPI
and p-MHPIC (see Figure 1b and Supporting Information
Scheme S1) were synthesized in good yield by the reaction with
aniline and the corresponding methoxy-substituted salicylalde-
hydes, respectively. To induce sufficient bathochromic shift for
green emission, the extended conjugation length strategy59 was
additionally implemented by the use of 9,10-phenanthrenequi-
none instead of benzil in p-MHPIC synthesis. HPNO, the
lowest energy emitting fluorophore in the imidazole series,59
was selected as a red emitting molecular pixel and synthesized
as described elsewhere.61 All new synthesized materials are fully
On the contrary, as total chromophore concentration
increased, a significant red-shift of emission was observed
from binary mixture solutions containing m-MHPI and a well-
1
characterized by H NMR, 13C NMR, HRMS, and elemental
analysis (see Experimental Section).
Steady-state UV−vis absorption and normalized fluorescence
emission spectra of the synthesized molecules in chloroform
solution and dye-doped polymethyl methacrylate (PMMA)
films are shown in Figure 2a and b, respectively. The details
including photoluminescence quantum yields (ΦPL) and
fluorescence lifetimes (τ) are also summarized in Table 1. As
we intended, m-MHPI and p-MHPIC showed blue (λmax = 461
known conventional red emitter (λmax,abs = 471 nm and λmax,PL =
565 nm in 10 μm CHCl3 solution) 4-(dicyanomethylene)-2-
methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM, see Sup-
porting Information Figure S4).62 Above 0.1 mM of total
concentration, the blue emission from m-MHPI at around 460
nm was almost completely quenched, whereas the red emission
from DCM at around 580 nm was noticeably enhanced (see
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Figure 3. (a) Emission color coordinates of the solution mixtures in the CIE 1931 chromaticity diagram: individual ESIPT dyes (★, m-MHPI, HPI,
○
△
p-MHPIC, and HPNO), desired values ( ), and experimentally obtained values of the mixtures ( , W1, C1, Y1, and M1). The inset shows
photographs of the individual ESIPT dyes and the corresponding mixtures under 330 nm UV light. Normalized emission spectra of the
corresponding solution mixtures: (b) W1, (c) C1, (d) Y1, (e) M1 at different concentrations (1 × 10−6 to 1 × 10−3 M). The black solid lines indicate
the expected spectra of the mixtures. The excitation wavelength was 330 nm.
Table 2. Mixing Ratios and the CIE Chromaticity Coordinates of the Mixture Solutions and the PMMA Doped Mixture Films
a
mixing ratio
chromaticity coordinates (x, y)
name
b
g
r
desired
obtained
solution
W1
C1
Y1
0.077
0.37
0
0.19
1.0
1.0
0
0.330, 0.330
0.205, 0.326
0.390, 0.458
0.320, 0.248
0.330, 0.330
0.190, 0.329
0.380, 0.434
0.320, 0.233
0.332, 0.330
0.204, 0.323
0.394, 0.457
0.322, 0.242
0.327, 0.332
0.192, 0.336
0.376, 0.436
0.318, 0.234
0.37
0
1.0
1.0
1.0
0
M1
W2
C2
Y2
0.11
0.019
1.0
film
0.015
0.91
0.023
0
0
1.0
1.0
M2
0.031
a
Mixing ratios of b:g:r are m-MHPI:p-MHPIC:HPNO in moles for the mixture solutions and HPI:p-MHPIC:HPNO in weight for the mixture films,
respectively.
Figure 2d). This is attributed to efficient energy transfer from
m-MHPI to DCM because the emission of m-MHPI and the
absorption of DCM have significant overlap (spectral overlap
integral, J(λ) = 1.25 × 1015 nm4 M−1 cm−1).63 It means that
conventional DCM molecules are very good energy acceptors
for K* emission of m-MHPI in the mixed solutions.
colors including cyan (C1), yellow (Y1), and magenta (M1)
within the triangle were selected to reproduce as representative
examples (see Figure 3a). A molar mixing ratio of m-MHPI:p-
MHPIC:HPNO = 0.077:0.19:1.0 was obtained to reproduce an
ideal white color W1 having chromaticity coordinates of (0.330,
0.330). In practice, mixing the three imidazole molecular pixels
produced a white emission with chromaticity coordinates of
(0.332, 0.330), which is almost identical to the desired color. In
the same way, the other secondary colors were successfully
reproduced with reasonable accuracy as summarized in Table 2.
It is worth noting that the spectral shape and the chromaticity
coordinates of the reproduced colors did not change at all,
while total chromophore concentration varied from 1 μM to 1
mM (see Figure 3b−e).
Importantly, the molecular pixel system functioned not only
in the solution but also in the solid thin films, which is crucial
for the real application. Interestingly, the imidazole molecular
pixels showed remarkably enhanced emission as compared to
that in the solution due to the bulky phenyl rings (see Table 1
and vide supra). The absolute photoluminescence quantum
yields (Φab,PL) of the films were 0.27, 0.44, and 0.014 for m-
MHPI, p-MHPIC, and HPNO, respectively. However, a slight
hypsochromic shift in the emission of the films as compared to
We evaluated molecular pixel behavior of the synthesized
ESIPT molecules by reproducing colors in the CIE 1931 XYZ
color space and the xy chromaticity diagram.11 The
chromaticity coordinates (x, y) of the imidazole molecules in
the solution are (0.167, 0.189) for m-MHPI, (0.260, 0.521) for
p-MHPIC, and (0.601, 0.356) for HPNO, respectively. As
shown in Figure 3a, the chromaticity coordinates of m-MHPI,
p-MHPIC, and HPNO correspond to blue, green, and red
colors of the CIE 1931 chromaticity diagram, respectively, and
form a color triangle called gamut that is the range of colors
that can be generated from additive combination of the three
emissions at its corners. According to Grassmann’s law, a
mixing ratio b:g:r for reproducing a certain desired color having
chromaticity coordinate of (x, y) can be easily calculated by
simple mathematical operations if the mixing components have
additive property (see the Supporting Information for the
operation details).11 A white color (W1) as well as secondary
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Figure 4. (a) Emission color coordinates of the ESIPT dye-doped PMMA film mixtures (total c = 1 wt %) in the CIE 1931 chromaticity diagram:
○
△
individual ESIPT dyes (★, m-MHPI, HPI, p-MHPIC, and HPNO), desired values ( ), and experimentally obtained values of the mixtures ( , W2,
C2, Y2, and M2). The inset shows the photographs of the individual ESIPT dyes and the corresponding mixtures under 340 nm UV light.
Normalized emission spectra of the corresponding film mixtures: (b) W2, (c) C2, (d) Y2, (e) M2. The excitation wavelength was 340 nm.
that in solution (vide supra) resulted in a small spectral overlap
between the emission of m-MHPI and the absorption of p-
MHPIC as well as that of HPNO (see Supporting Information
Figure S5). It means that ground-state E form of both p-
MHPIC and HPNO turned into a potential energy acceptor for
the K* emission of m-MHPI in the film state. As a result,
despite fairy small overlap (J(λ) ≈ 1 × 1012 nm4 M−1 cm−1, see
the Supporting Information), it was observed that the blue
emission from m-MHPI at around 440 nm was significantly
decreased, while the green and red emissions from p-MHPIC
and HPNO at around 500 and 610 nm were non-negligibly
enhanced due to partial energy transfer from m-MHPI to p-
MHPIC and HPNO in their simple binary mixture films (m-
MHPI:p-MHPIC and m-MHPI:HPNO = 1:1 in weight),
respectively (see Supporting Information Figure S6).
Fortunately, however, such inappropriate blue molecular
pixel for full-color reproduction in the film state could be
replaced by HPI. Because of the hypsochromic shift of the
emission in the PMMA film, the HPI-doped film showed
appropriately blue-shifted emission color in the CIE diagram,
which made no spectral overlap with p-MHPIC and HPNO
(see Figure 4a and Supporting Information Figure S5). The
chromaticity coordinates (x, y) of the imidazole-doped films are
(0.150, 0.156) for HPI, (0.229, 0.499) for p-MHPIC, and
(0.575, 0.350) for HPNO, respectively. Through the exact same
procedures as in the solution, representative colors including a
white (W2), cyan (C2), yellow (Y2), and magenta (M2) within
the gamut were successfully reproduced in the films (see Figure
4a and Table 2). Moreover, it should be noted that all of the
reproduced emission spectra were exactly matched with
predicted ones, which unambiguously confirmed their molec-
ular pixel behavior (see Figure 4b−e).
were prepared in 1:1 weight ratio: (1) HPI and p-MHPIC
(HP11); (2) HPI and HPNO (HO11); (3) p-MHPIC and
HPNO (PO11); (4) m-MHPI and p-MHPIC (MP11); and (5)
m-MHPI and HPNO (MO11), respectively. In the conven-
tional resonant energy transfer system, the presence of acceptor
molecules shortens the fluorescence lifetime of donor
molecules.13 In contrast, HPI being an ineffective energy
donor exhibited almost unaltered decay profiles (χ2 = 1.04)
with lifetimes of 2.42 ns (33.6%) and 4.02 ns (66.4%) in the
HO11 film when monitored at 500 nm. Similarly, despite the
presence of excess amount of HPNO, p-MHPIC did not show
any significant lifetime changes in the PO11 film with time
constant of 1.63 ns (26.6%) and 5.00 ns (73.4%) when
monitored at 520 nm (χ2 = 1.05). Moreover, even slightly
lengthened lifetime decay profiles of a possible donor
component were observed in the HP11 film. The measured
time constants of HPI in the film were 2.91 ns (57.3%) and
4.24 ns (42.7%) at 420 nm (χ2 = 1.01). On the contrary,
shortened lifetime decay profiles of m-MHPI obtained in both
MP11 (1.39 and 2.81 ns at 420 nm) and MO11 (1.58 and 2.90
ns at 500 nm) films indicate the presence of energy transfer
from m-MHPI to p-MHPIC and HPNO, respectively,
consistent with the steady-state results stated above (see
Supporting Information Figures S11−15 for decay profiles).
CONCLUSIONS
■
We have successfully constructed a molecular pixel system for
full-color reproduction consisting of three primary color-
emitting imidazole ESIPT molecules between which energy
transfer was totally frustrated. The whole color gamut within
the color triangle on the CIE 1931 diagram could be easily
predicted and precisely reproduced by the simple mathematical
operations based on additive color theory. It is expected that
this unprecedented demonstration of molecular pixel system
will pave the way to the innovative optoelectronic applications
in future displays and biological imaging.
Finally, fluorescence lifetime measurements could underpin
that the molecular pixel behavior of the films is attributed to the
frustrated energy transfer between the ESIPT molecules. As
summarized in Table 1, all of the four imidazole molecules
exhibited biexponential decay in PMMA films (see Supporting
Information Figures S7−10 for decay profiles). To check
fluorescence lifetime changes, five different binary mixture films
11244
dx.doi.org/10.1021/ja404256s | J. Am. Chem. Soc. 2013, 135, 11239−11246

Journal of the American Chemical Society
Article
(26) Tseng, K. P.; Fang, F. C.; Shyue, J. J.; Wong, K. T.; Raffy, G.;
Del Guerzo, A.; Bassani, D. M. Angew. Chem., Int. Ed. 2011, 50, 7032.
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(28) Liu, J.; Xie, Z. Y.; Cheng, Y. X.; Geng, Y. H.; Wang, L. X.; Jing,
X. B.; Wang, F. S. Adv. Mater. 2007, 19, 531.
ASSOCIATED CONTENT
* Supporting Information
Calculation details for mixing ratios and spectral overlap
integrals, synthetic schemes, supporting figures, and lifetime
decay profiles. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
S
(29) Zhang, K.; Chen, Z.; Yang, C.; Tao, Y.; Zou, Y.; Qin, J.; Cao, Y.
J. Mater. Chem. 2008, 18, 291.
(30) Park, M. J.; Kwak, J.; Lee, J.; Jung, I. H.; Kong, H.; Lee, C.;
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4284.
AUTHOR INFORMATION
Corresponding Author
parksy@snu.ac.kr
■
Notes
(32) Baier, M. C.; Huber, J.; Mecking, S. J. Am. Chem. Soc. 2009, 131,
14267.
The authors declare no competing financial interest.
(33) Chen, J.; Zeng, F.; Wu, S. Z.; Su, J.; Tong, Z. Small 2009, 5, 970.
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ACKNOWLEDGMENTS
■
This research was supported by a National Research
Foundation of Korea (NRF) grant funded by the Korea
government (MSIP) (no. 2009-0081571).
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